Computer numerical controlled or CNC machines are sophisticated metalworking tools that can generate intricate parts required by modern day technology. The CNC term refers to a large group of machines that utilize computer logic to control movements of a cutting head or spindle or even the work piece itself to fulfill the metalworking task. CNC machines may commonly include milling machines, lathes, and grinders.
Milling machines, for example, automatically cut materials, including metal and today's advanced carbon composites, using a machine spindle affixed with a milling cutter, which can effectively move to different positions and depths as directed by the computer instructions. The milling cutter is a rotary cutting tool, often comprising multiple cutting points, and typically travels perpendicular to its axis so that the cutting action primarily occurs relatively about the circumference of the milling cutter. As the milling cutter enters the work piece, the cutting edges, generally in the form of flutes or teeth, repeatedly cut into and exit from the material, such to emit chips or a spring-like swarf therefrom with each working pass. The cutting action in this regard is often referred to as shear deformation.
Nearly all milling machines, from the oldest manual machines up to the most modern CNC machines, utilize tooling that is piloted on a tapered surface. The machine taper is a simple, low-cost, highly repeatable, and versatile tool mounting system that utilizes a tool holder with a progressively tapered shank and a geometrically matching socket interiorly present in the machine spindle. In most cases, the tool holder is retained within the spindle's socket by means of a frictional force. However, in some cases, the milling machine spindle may unduly experience an increased load that can perhaps overwhelm the holding frictional force. Accordingly, it is appropriate to use a drawbar, which is essentially a long bolt that retains the tool holder within the spindle's socket by means of establishing a sufficient amount of upward force that overcomes the transverse force component that would otherwise cause the tool holder to wobble out of the spindle's socket.
In generally all instances of CNC machine usage, it is vital that the interchangeable interface between a machine spindle and a cutting tool is efficiently maintained so as not to deleteriously impact the accurate milling of materials. Accordingly, in fulfilling the requisite amount of efficiency, it is appropriate to evaluate four factors: (1) the rotational axis of the machine spindle and cutting tool to ensure a high degree of concentricity in relation to each, (2) the cutting tool's position and hold within the tool holder to resist unwanted rotational motion therewithin and axial pullout therefrom, (3) the consistency by which the tool holder is fabricated from manufacturer to manufacturer, which can be effectively addressed by the application and use of gages that have become standardized in the industry, and (4) the degree by which the tool holder is relatively balanced with respect to the machine spindle.
Industrial manufacturers of tool holders, for instance, have made numerous efforts to satisfactorily develop tool holders addressing the efficiency requirements duly noted above, but generally in view of making tradeoffs among cost, versatility of use, long-term accuracy, and maintenance.
For instance, industry has come to popularize and accept standard collets that allow the tool holder to be more readily adaptable to a variety of cutting tool diameters while yielding a moderate degree of concentricity, at least initially. Generally, the standard collet can be described as comprising a tubular body formed from a plurality of elongated, flexible steel members. The members are separated by longitudinal slots that impart some degree of radial flexibility to the collet so as to further advance a grip on the cutting tool's shank. Adjacent gripping members are interconnected by an alternating pattern of metal webs to form a singular collet body. In the manner of operation, the collet body is inserted in a complementary-shaped opening in a collet chuck so that an exposed portion thereof extends beyond the collet chuck. An annular collet nut having an inner diameter screw thread that corresponds to an outer diameter screw thread on the collet chuck is then placed over the collet chuck and exposed portion of the collet body. The collet nut includes a nose ring with a frustro-conical cam surface that engages the exposed portion of the collet body and squeezes it radially inward as the collet nut is tightly screwed onto the collet chuck. The radial compression that the collet nut applies to the exposed portion of the collet body flexes the body inwardly, appreciably creating a sufficient gripping force to retain placement of the cutting tool's shank within the inner diameter of the collet body.
Although the collet nut and associated tapers of the collet and collet chuck portion of the tool holder may adequately compress and clamp the cutting tool's shank within the collet body, the strength with which the collet clamps the cutting tool may become unduly compromised by the applied rotational force and rotational resistance that may be realized when the cutting tool engagingly shears the work piece during milling operations. Since the respective shapes of the cutting tool's shank and the inner portion of the collet body of the tool holder are inherently cylindrical by design, there is an appreciable tendency for rotational slip of the cutting tool relatively within the tool holder, particularly under a condition of high operating torque for a sustained period of time during rigorous milling operations. Consequently, unwanted rotational interaction of this kind can adversely cause axial pullout of the cutting tool from the tool holder, leading to possible damage to the tool holder, machine spindle, cutting tool, and/or work piece, and/or further an unbalanced tool holder, resulting in unwanted vibrations that can create chatter and ultimately diminish the surface finish of the work piece and the life of the cutting tool. Accordingly, to avoid rotational slip and axial pullout in the manner described herein, feed rates and operational RPM must be unduly adjusted or restricted to the point of possibly becoming impractical, and in today's fast paced manufacturing environment, this becomes even more of a concern. Therefore, an improved form of machine tool holding apparatus that prevents tool rotation and axial pullout is particularly desired in this respect.
Although the usage of a tool holder incorporating a collet may be satisfactory in some regards, particularly where variability thereof is required to accommodate the variety of cutting tools being offered by and used in the industry, industry has come to recognize other forms of tool holders that sufficiently overcome the deficiencies associated with collets, but perhaps compromising in areas of versatility of use and expense.
For instance, tool holders that incorporate clamping mechanisms in the form of hydraulics or heat shrinking, can effectively eliminate the use of the collet arrangement described above and achieve a relative amount of holding power with a high degree of concentricity. This is generally made possible by the nature of the unibody structure of the tool holder that is fabricated to an overall tight tolerance to further a low run-out to improve tool precision, surface finish, and productivity.
Hydraulic tool holders, for example, incorporate a bladder of hydraulic fluid that fulfill clamping upon the tool's shank with two supports on each side (fulcrums); tool change out is made possible by means of tightening and loosening a set screw with a dedicated torque wrench that respectively pressurizes and depressurizes the bladders. Although a relative degree of concentricity is realized with a fulfilling option for tool change out to accommodate a variety of cutting tool diameters, possibly to the likes of the collet arrangement, there continues to be an undesirable opportunity for axial slip and pullout of the cutting tool. A shrink fit tool holder, on the other hand, can offer an appreciable amount of clamping force upon the tool's shank while maintaining a high degree of concentricity, as it relies on the heating and cooling of the tool holder's metal construction to take advantage of thermal expansion and contraction that respectively holds and releases the cutting tool within and from the tool holder. Although a shrink-fit tool holder provides a simple yet highly accurate and repeatable grip on the cutting tool, it generally suffers in the area of versatility given that the cutting tool is generally dedicated to the tool holder unless provisions are made to purchase, operate and maintain a heat-shrink machine that can accommodate periodic change out of the cutting tool, which can be a time consuming and costly and expensive option in the long term.
Accordingly, there remains a need for a simple, yet cost effective tool holding apparatus that incorporates an internal clamping mechanism that aims to eliminate any undesirable opportunity for axial rotation and axial pullout that can unduly diminish the precision and accuracy by which the cutting tool shapes and mills a stock of material, while simultaneously maintaining a high degree of concentricity relatively to the centerline of the machine spindle that is accurate to a level of run out error appropriate to the cutting tool and the machining process.